There is an ongoing debate about whether cover letters have any value. People who like to dress casually often have a tie rolled up in their pocket when they head to a nice place for dinner. They may need it or they may not. Doing a cover letter for every customized resume can’t hurt and it might help. It’s like the tie. The place you want to be may want it or may not. You might as well include it in the package. As an added bonus, it’s another step in preparing for the interview. Everything that goes into the cover letter is likely to come up in the interview, if you’re getting the most you can out of the interview.